1. Technical Field
A phase locked loop (abbreviated as “PLL”) is disclosed. More specifically, a PLL having a voltage control oscillator (abbreviated as “VCO”) configured to divide the frequency outputted from the VCO in wide frequency bands is disclosed.
2. Description of the Related Art
FIG. 1 is a block diagram illustrating a conventional PLL using a pulse swallow system.
The conventional PLL comprises a phase comparator 1, a low pass filter 2, a VCO 3, a dual modulus prescaler 4, a program counter 5, a swallow counter 6 and a controller 7. The phase comparator 1 compares a reference frequency fr of an external clock signal with a comparison frequency fp of a comparison clock signal PCLK. The VCO 3 generates a clock signal ICLK. The frequency of the clock signal ICLK is proportional to the direct current (DC) signal from the low pass filter 2. The dual modulus prescaler 4 divides a frequency of the clock signal ICLK in the ratios of 1/M and 1/(M+1). The program counter 5 divides a clock signal divided by the prescaler 4 in the ratio of 1/N. The swallow counter 6 divides the clock signal divided by the prescaler 4 in the ratio of 1/A. The controller 7 outputs a mode control signal MC for controlling the prescaler 4 using signals from the program counter 5 and the swallow counter 6.
A frequency fvco of the clock signal ICLK from the VCO 3 is divided by the dual modulus prescaler 4 in the ratios of 1/M and 1/(M+1), and then inputted into the program counter 5 and the swallow counter 6.
The swallow counter 6 is used to control the division ratio of the prescaler 4. The prescaler 4 divides in the ratio of 1/(M+1) until the swallow counter 6 counts A pulses.
After the swallow counter 6 counts A pulses, the prescaler 4 divides in the ratio of 1/M. The comparison frequency fp of the comparison clock signal PCLK is defined by Equation 1.                                                         fp              =                              fvco                                  {                                                            (                                                                        (                                                                                    (                                                              M                                +                                1                                                            )                                                        ×                            N                                                    )                                                ×                                                  A                          N                                                                    )                                        +                                          (                                                                        (                                                      M                            ×                            N                                                    )                                                ×                                                                              (                                                          N                              -                              A                                                        )                                                    N                                                                    )                                                        }                                                                                                        =                                                fv                  ⁢                                                                           ⁢                  co                                                  {                                                            (                                                                        (                                                      M                            +                            1                                                    )                                                ×                        A                                            )                                        +                                          (                                                                        (                                                      N                            -                            A                                                    )                                                ×                        M                                            )                                                        }                                                                                        Equation        ⁢                                   ⁢        1            
The output frequency fvco of the output clock signal ICLK is defined by Equation 2 (here, fp=fr).                                                         fvco              =                              fp                ⁢                                  {                                                            (                                                                        (                                                      M                            +                            1                                                    )                                                ×                        A                                            )                                        +                                          (                                                                        (                                                      N                            -                            A                                                    )                                                ×                        M                                            )                                                        }                                                                                                        =                              fp                ⁡                                  (                                      A                    +                                          M                      ×                      N                                                        )                                                                                                        =                              fr                ⁡                                  (                                      A                    +                                          M                      ×                      N                                                        )                                                                                        Equation        ⁢                                   ⁢        2            
In Equation 2, When the value of A varies, the frequency fvco of clock signal ICLK is changed. In this way, if the prescaler 4 is used in the PLL, the channel separation can be the reference frequency fr. Particularly, in a frequency synthesizer of high frequency, a pulse swallow system is used because the prescaler 4 is set in a high division ratio.
Generally, the output frequency fvco of the pulse swallow system is defined by the following equation 3:                     fvco        =                              {                                          (                                  M                  ×                  N                                )                            +              A                        }                    ×                      fosc            R                                              Equation        ⁢                                   ⁢        3            
Here, M is the division ratio of the prescaler 24, and N is the value of the program counter 25. A is the value of the swallow counter 26, which is smaller than N. In Equation 3, fosc represents the reference oscillating frequency, and R represents the value of the reference counter.
The VCO of the conventional PLL having a pulse swallow system cannot use various frequency bands due to its non-linear characteristic.